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<ul><li><code>Cookie、LocalStorage和 IndexDB</code>无法读取</li> <li><code>DOM 和 JS</code>对象无法获取</li> <li><code>AJAX</code>请求不能发送</li></ul></li></ul> <h2 id="_1-jsonp-跨域"><a href="#_1-jsonp-跨域" class="header-anchor">#</a> 1. JSONP 跨域</h2> <ul><li>原理是通过<code>&lt;script&gt;</code>标签没有跨域限制。</li> <li>后端返回数据的格式是<code>showData(result)</code>, 因为是<code>javacript</code>脚本，所以会自动调用<code>showData</code>函数，即<code>window.showData</code>。</li> <li>只支持<code>Get</code>请求。</li></ul> <div class="language-html extra-class"><pre class="language-html"><code>  <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span><span class="token punctuation">&gt;</span></span><span class="token script"><span class="token language-javascript">
    <span class="token comment">// 向头部输入一个脚本，该脚本发起一个跨域请求</span>
    <span class="token function">$</span><span class="token punctuation">(</span><span class="token string">&quot;head&quot;</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">&quot;&lt;script src='http://localhost:9090/student?callback=showData'&gt;&lt;\/script&gt;&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">function</span> <span class="token function">showData</span><span class="token punctuation">(</span><span class="token parameter">result</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'这就是返回的数据：'</span><span class="token punctuation">,</span> result<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
  </span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
</code></pre></div><div class="language-java extra-class"><pre class="language-java"><code>  <span class="token comment">// 前端传过来的回调函数名称</span>
  <span class="token class-name">String</span> callback <span class="token operator">=</span> request<span class="token punctuation">.</span><span class="token function">getParameter</span><span class="token punctuation">(</span><span class="token string">&quot;callback&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// 用回调函数名称包裹返回数据，这样，返回数据就作为回调函数的参数传回去了</span>
  result <span class="token operator">=</span> callback <span class="token operator">+</span> <span class="token string">&quot;(&quot;</span> <span class="token operator">+</span> result <span class="token operator">+</span> <span class="token string">&quot;)&quot;</span><span class="token punctuation">;</span>
  response<span class="token punctuation">.</span><span class="token function">getWriter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><ul><li><code>jQuery</code>式的<code>jsonp</code></li></ul> <div class="language-javascript extra-class"><pre class="language-javascript"><code>  $<span class="token punctuation">.</span><span class="token function">ajax</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
      url<span class="token operator">:</span> <span class="token string">&quot;http://localhost:9090/student&quot;</span><span class="token punctuation">,</span>
      type<span class="token operator">:</span> <span class="token string">&quot;GET&quot;</span><span class="token punctuation">,</span>
      dataType<span class="token operator">:</span> <span class="token string">&quot;jsonp&quot;</span><span class="token punctuation">,</span>  <span class="token comment">// 指定服务器返回的数据类型</span>
      jsonp<span class="token operator">:</span> <span class="token string">&quot;theFunction&quot;</span><span class="token punctuation">,</span>   <span class="token comment">// 指定参数名称</span>
      jsonpCallback<span class="token operator">:</span> <span class="token string">&quot;showData&quot;</span><span class="token punctuation">,</span>  <span class="token comment">// 指定回调函数名称</span>
      <span class="token function-variable function">success</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">data</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
          console<span class="token punctuation">.</span><span class="token function">info</span><span class="token punctuation">(</span><span class="token string">&quot;调用success&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// 最终会拼成以下这样</span>
  <span class="token comment">// ===&gt; http://localhost:9090/student?theFunction=showData</span>
</code></pre></div><h2 id="_2-跨域资源共享（cors）-cross-origin-resource-sharing"><a href="#_2-跨域资源共享（cors）-cross-origin-resource-sharing" class="header-anchor">#</a> 2. 跨域资源共享（CORS） Cross-origin-resource-sharing</h2> <ul><li>允许浏览器向跨源服务器，发起XMLHttpRequest请求</li> <li>简单请求
<ul><li>请求头部加一个字段：<code>Origin</code></li> <li>服务器响应头部要求设置的字段
<ul><li>必要的字段：<code>Access-Control-Allow-Origin</code>.允许跨域接受的域名(*<em>若没有包含Origin里的字段的，说明请求报错</em>)</li> <li>可选字段：<code>Access-Control-Allow-Credentials</code>.是否允许头部带<code>Cookie</code>值，默认不带。若要带<code>Cookie</code>（同时请求时<code>withCredentials</code>需要设置为true,且<code>Allow-Origin</code>不能为<code>*</code>号）</li> <li>可选字段：<code>Access-Control-Expose-Headers</code>,能够从响应头部获取值的相关字段。默认只能去6个（Cache-Control、Content-Language、Content-Type、Expires、Last-Modified、Pragma），要获取其他则需要配置</li></ul></li></ul></li> <li>非简单请求
<ul><li>其他的请求方法（<code>PUT</code>/<code>DELETE</code>）</li> <li>预检请求（<code>options</code>请求方法）</li> <li>请求头需要设置的字段
<ul><li><code>Origin</code></li> <li><code>Access-Control-Allow-Methods</code></li> <li><code>Access-Control-Request-Headers</code> 若请求头有这个字段，则响应头必须有<code>Access-Control-Allow-Headers</code></li> <li><code>Access-Control-Max-Age</code>: 表示此次预请求的有效时间，在此期间不会再发预请求。</li></ul></li></ul></li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">最后一次修改时间:</span> <span class="time">9/28/2020, 11:37:23 AM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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